1. Field of the Invention
The present invention relates to an organic EL (Electro Luminescence) element drive circuit and an organic EL display device and, in particular, the present invention relates to an organic EL element drive circuit of a simple matrix type organic EL panel used in a portable telephone set, etc., which is capable of reducing luminance variation on a screen of an organic EL display device due to difference in characteristics between current driver ICs and is suitable for a high luminance color display and the organic EL display device.
2. Description of the Related Art
Since an organic EL display device can perform a high luminance display due to spontaneous light emission, the organic EL display device is suitable for use in a display device whose display screen size is small and is expected as the next generation display device to be mounted on such as a portable telephone set, a DVD player or a PDA (personal digital assistance), etc. A known problem of the organic EL display device is that variation of luminance becomes considerable when a voltage drive is applied to the organic EL display device as in a liquid crystal display device and the drive control becomes difficult due to the difference in sensitivity between R (red), G (green) and B (blue).
In view of this problem, an organic EL display device using a current driver is proposed recently. For example, in JP H10-112391A, a technique for solving the problem of luminance variation by employing the current drive is disclosed.
In a recent organic EL display panel of an organic EL display device for use in a portable telephone set, the number of terminal pins of column lines is 396 (132×3) and the number of terminal pins of row lines is 162. These numbers of the terminal pins are still increasing.
With such increase of the number of terminal pins, a plurality of column IC drivers is three currently and the number of terminal pins of each driver for one of R, G and B in the case of full color display is 44, so that the total number of the terminal pins of the three drivers becomes 132. Therefore, there is a problem that luminance variation occurs on a screen of an organic EL display device due to difference in characteristics between the column IC drivers, particularly, due to variation of drive circuits thereof.
In, for example, U.S. application Ser. No. 10/102,671, which corresponds to Japanese Application JP 2002-82662 claiming domestic priorities of Japanese Application JP2001-86967 and JP2001-396219, a technique for solving this problem is disclosed.
Further, JP H2001-42827A discloses another technique for solving the above problem.
In U.S. application Ser. No. 10/102,671, in order to prevent luminance variation due to difference in characteristics between a plurality of column IC drivers, a drive stage is constructed with a current mirror circuit including an input side transistor and a plurality of parallel-connected output side transistors, so that drive currents for column pins are generated. By arranging the input side transistor of the current mirror circuit at a center of the output side transistors thereof, the pin drive current flowing through the first output pin of the column IC driver is made substantially equal to that flowing the last output pin. Further, the pin drive current flowing the last output pin in a certain column IC driver and the pin drive current flowing the first output pin of a next column IC driver are regulated by selecting resistance values thereof by means of laser trimming such that these pin drive currents become specific values with which the drive current characteristics of the column IC drivers become equal and the luminance variation problem is solved.
On the other hand, in order to solve the problem of the difference in characteristics between the column IC drivers, JP H2001-42827A utilizes a current mirror circuit having similar construction to that in U.S. application Ser. No. 10/102,671. However, an output current of the last output side transistor of the current mirror circuit is derived externally of the IC and inputted to a next column IC driver such that the drive current of the input side transistor is made equal to that of the input side transistor of the first column IC driver. Although the pin drive currents of the column IC drivers may be made substantially equal, it is practically difficult to effectively utilize this technique.
FIG. 2 is a circuit diagram disclosed in JP H2001-42827A. In FIG. 2, an initial stage column IC driver (first anode line drive circuit) 21 includes a reference current control circuit RC, a control current output circuit CO, a switch block SB having switches S1 to Sm, and m current drive sources provided correspondingly to respective pins. The m current drive sources are constructed with transistors Q1 to Qm and resistors R1 to Rm, respectively. A next stage column IC driver (second anode line drive circuit) 22 includes a drive current output circuit CC, a switch block including switches S1 to Sm and m current drive sources provided correspondingly to respective pins. The m current drive sources are constructed with transistors Q1 to Qm and resistors R1 to Rm, respectively. Output currents i of the transistors Q1 to Qm of the drivers are supplied to the pins through the switches S1 to Sm and output terminals X1 to Xm, respectively.
The reference current control circuit RC is constructed with an operational amplifier OP supplied with a reference voltage VREF, a transistor Qa, which is driven by an output of the operational amplifier OP supplied to a base thereof, resistor Rp provided between an emitter of the transistor Qa and ground and a transistor Qb having a collector, which is connected to a collector of the transistor Qa on an upstream side of the transistor Qa. A voltage generated by the resistor Rp is fedback to an input of the operational amplifier OP, so that the reference current control circuit constitutes a constant current source. An emitter of the transistor Qb is connected to a power source line VBE (corresponding to a power source line VDD of the display device) through a resistor Rr.
The transistor Qb constitutes an input side current mirror circuit together with the transistors Q1 to Qm and a transistor Qo of the control current output circuit CO and is driven by a reference current IREF generated by the reference current control circuit RC.
The drive current output circuit CC of the driver 22 corresponds to the reference current control circuit RC. The drive current output circuit CC is constructed with a current mirror circuit including transistors Qc and Qd and a transistor Qe driven by the output side transistor Qd of the current mirror circuit. The input side transistor Qc of the driver 22 is supplied with an output current Iout of the control current output circuit CO of the driver 21, which is ic, to drive the transistor Qe of the driver 22. The transistor Qe of the driver 22 is an input side transistor of the transistors Q1 to Qm constituting a current mirror circuit. Values of the resistors Ro and Rr are equal and a value of the resistor Rs is equal to a value of the parallel resistors R1 to Rm. The switches S1 to Sm of the switch block SB of the driver 21 are ON/OFF controlled by control signals GA1 to GAm and the switches S1 to Sm of the switch block SB of the driver 22 are ON/OFF controlled by control signals GB1 to GBm.
As another circuit construction, a current drive circuit is provided in a position corresponding to the switch block SB in each of the drivers. In the current drive circuit, input side transistors are provided correspondingly to terminal pins and a pair of current mirror current output circuits having output side transistors connected to terminal pins are provided. The switching operation of the current drive circuit is ON/OFF controlled by the control signals GA1 to GAm. In this circuit, the current mirror output circuit becomes a drive stage, which generates the mirror currents correspondingly to the terminal pins according to a reference current from a reference current generator circuit (corresponding to the reference current control circuit RC) as an input stage preceding to the drive stage. Alternatively, the mirror currents distributed to the terminal pins are amplified by k times (k is an integer equal to or larger than 2) and drive the output circuits.
In the current drive circuit disclosed in Japanese Application JP2002-82662, D/A converter circuits are provided correspondingly to the terminal pins as the k-time amplifier circuits. The D/A converter circuits receive display data corresponding to the column side terminal pins and the column side drive currents for the respective terminal pins for one line are generated simultaneously by A/D converting the column data.
A problem of the current drive circuit in which the current mirror circuit for parallel driving a plurality of output side transistors is used in the drive stage or the output stage will be described with reference to the IC driver circuits 21 and 22 shown in FIG. 2.
In the circuit shown in FIG. 2, the output current Iout=ic of the transistor Qo of the column IC driver circuit 21 is supplied to the transistor Qe of the column IC driver circuit 22 through the current mirror transistors Qc and Qd. Therefore, the output current should be the current i equal to the reference current IREF theoretically. However, even if the reference currents are made equal between chips in this manner, characteristics (hfe and Early voltage, etc.) of transistors of the converter circuit and the output circuit are different between chips. Therefore, it is difficult to make actual output currents precisely coincident between chips. Further, since the reference current i is generated by the column IC driver 22 upon reception the current Iout, which is one of the output currents of the column IC driver 21, a difference between the reference current i of the column IC driver 22 and the reference current IREF of the column IC driver 21 becomes large, so that the luminance variation in the boarder region between the drivers can not be removed sufficiently.
In this view point, the technique disclosed in Japanese Application JP2001-86967 can remove such luminance variation since the drive currents in specific positions are regulated for every driver unlike the case where the output current (drive current) sent as shown in FIG. 2. However, since, in JP2001-86967, it is necessary to select the values of resistors to be trimmed by laser trimming for every column IC driver during the fabrication of the drive circuit, there is another problem that the fabrication efficiency is lowered.